Monographs of the USGS Vol. XLVI
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composed of stout rectangular feldspars, with a somewhat below the falls, and then tracing it back into the finer rounded outline, and internally changed to saussurite, though grained and more compact part near the bridge.” their periphery is mostly clear. Between these are the remains of former hornblende (possibly pyroxene) individuals now aWilliams, op. cit., p. 83. represented only by amphibole fibers and chlorite. Beautiful bIbid., p. 94. skeleton forms of leucoxene, composed of three sets of c parallel bands reproducing the rhombohedral parting of the Wadsworth, M. E., Report of State Board of Geological Survey for original ilmenite, are abundant.b 1891 and 1892, Lansing, 1890, p. 125. The rock of the schist band is much more altered. Its Origin of the schistosity.—The least-altered phases of feldspar is mostly changed to calcite, and the the fine-grained greenstones are characterized by a hornblende to chlorite. The original structure has wholly rhomboidal parting. As the rocks depart more and more disappeared, and there is now a fine mosaic of quartz from their original character the rhombs produced by the and secondary albite between the calcite and chlorite parting become more and more elongated until finally masses. The same skeleton forms of leucoxene remain, their sides are approximately parallel and a well- however, and “there is no doubt developed schistosity results. At the same time the original components of the rock mass undergo change. a Williams,, op. cit, pp. 77-78. In many of the less-altered schists that have been bIbid., p. 83. derived from massive eruptives the structure of the originals can still be recognized, though their mineral that the two specimens represent the same rock in composition can only be surmised, since they consist of different stages of alteration, the more changed form secondary products exclusively. having become decidedly schistose.”a Basic lavas and their derived schists.—In the field, dark- FINE-GRAINED VARIETIES. and light-colored schists are readily distinguished, and The finer-grained greenstones and greenstone-schists both are closely associated with nearly massive phases. were originally diabases, basalts, and basic tuffs. In On the east side of the basin, below the Little Quinnesec composition they are similar to the coarser-grained Falls, the dark schists are so intimately associated with varieties described. Their structure is less well defined, massive greenstones that there can be little doubt as to and in some cases the original rock appears to have the original identity of the two. In the hand specimen the been essentially a glass. One of the best preserved of least schistose rock is a compact, aphanitic mass of a the fine-grained rocks is that on the Michigan side of the dark-green color. Little Quinnesec Falls. It is a massive, dark-green rock Under the microscope the original diabasic nature of the rock is which, according to Williams— at once apparent, although the extensive mineralogical was originally a diabase, although its present constituents are changes which have gone on have greatly obscured its former for the most part secondary. The shapes of the original structure. The components now present are almost wholly minerals are indistinctly outlined, and so the structure of the secondary. These are hornblende, chlorite containing epidote, rock is preserved. There is now present a pale-green quartz, and leucoxene. Ilmenite and occasional traces of hornblende, probably secondary to pyroxene, although no feldspar are the only original constituents which remain. Still, traces of this mineral are preserved; epidote, chlorite, the disposition of the secondary minerals is such as to outline saussurite, and leucoxene in zones around the titanic iron. a diabasic or ophitic structure often with great distinctness. The feldspar has rarely changed to the opaque, gray The feldspar is rarely well preserved; but a narrow zone of the saussurite, but is replaced for the most part by a mass of unaltered substance of this mineral often outlines a stoutly lath- sharply defined epidote crystals. Where the feldspar and shaped crystal, even when its interior is wholly changed to an hornblende have jointly contributed to the formation of aggregate of quartz, chlorite, and epidote. The hornblende has secondary products, we have the chlorite-epidote aggregate as a curious appearance. Its crystals are brownish and a result. A little secondary quartz is also observable.b nonpleochroic with a somewhat granulated surface, so that it externally resembles diallage. Its cleavage and optical These massive or almost massive rocks, as has been properties prove it to be undoubtedly hornblende, although this said, grade in many places into schists. Where they do superficial likeness to diallage is so strong as to almost compel not actually pass into schistose phases they are often the conviction that it has originated by paramorphism from a associated with basic schists in which traces of the pyroxene. This brown hornblende is seen with a high power to original structure of eruptives can be detected or which be gradually changing to a green variety, in which a pleochroism for the first time becomes apparent. This also can be traced into rocks exhibiting this structure. In c frequently passes over into a fibrous hornblende. The chlorite- other eases, as stated by Wadsworth in connection with epidote aggregate in these rocks is very finely developed. The the discussion of the origin of these rocks at the Little chlorite is of an emerald-green color and distinctly pleoehroic. and the Big Quinnesec Falls, the basic schists may have It appears between crossed nicols as isotropic or polarizes with been derived from volcanic tuffs or, as Wadsworth calls a maroon tint. The epidote is in sharp, light-yellow crystals, them, porodites. The predominant rock on the Michigan with the characteristic shape and optical properties of this side of the lower fells, for instance, is plainly species. * * * This chlorite-epidote aggregate covers conglomeratic. It is either an old eruptive ash or an considerable areas and occupies the place of both the agglomerate. “Of the fraginental and conglomeratic feldspathic and the pyroxenic constituents. In addition to the minerals already named, ilmenite with its leucoxene border, nature of the rock,” Wadsworth declares, “no one can pyrite, and secondary quartz are quite abundant in these possibly doubt after studying it on the Michigan side, rocks.a Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 54 of 111 Through this rock passes a schistose band so intimately structure when this has not been completely destroyed. related to the massive rock in both sides of it that there Usually, however, the tuffaceous structure disappears so can not be the least doubt as to the continuity of the two. rapidly with the assumption of schistosity that nothing The hand specimen taken from the band is decidedly remains to distinguish the schists derived from tuffs from schistose. those derived from fine-grained eruptives. Under the microscope it shows the effects of mechanical In a few instances sections made from the schists at the crushing and attendant mineralogical changes with great Little Quinnesec Falls possess characteristics that point distinctness. The whole rock, with the exception of certain to a tuffaceous origin. In natural light these sections remains of the larger feldspar crystals, has been reduced to a show irregular granular areas and black dots in a fine-grained mass, showing an aggregate polarization. Light- colorless, transparent, almost amorphous matrix. green chlorite has been largely developed and has completely replaced all the bisilicate elements. The parallel arrangement Between crossed nicols plagioclase, saussurite, kaolin, of the scales of this mineral is what produces the schistose calcite, colorless or light-green chlorite, and bleached structure.b hornblende are noticeable. The plagioclase is largely altered into chlorite, saussurite, and kaolin. The greater The chlorite together with clear grains of quartz and portion of the mineral is often fresh enough to exhibit probably of unstriated feldspar are embedded in a clearly its twinning striations, though not fresh enough to quartz-albite mosaic. Calcite is also abundant as little yield accurate measurements of extinction angles. This nests in the mosaic, and rutile either in stout yellow feldspar occurs in lath-shaped crystals, in rounded and grains, or in minute sharp crystals traverse it in long irregular-shaped grains, in sharp-edged fragments, and sinuous lines. This rutile represents the ilmenite of the in some instances it appears as though it were a original rock. secondary deposit filling triangular spaces between the The mechanical action which produced this schistose band has other components. Some of the crystals are fairly well therefore resulted in the crushing of the rock, and the almost formed, others are broken, and still others are total disappearance of all of the original components. The castellated at their ends. A large number of skeleton comparatively slight change in the chemical composition of the crystals are also noticed scattered through the matrix. rock as a whole may be seen from the two following analyses * * * made by Mr. R. B. Riggs: aWilliams, op. cit., p. 91. Only a few indications of the original structures can be detected. In a few small areas of some of the sections the structure appears to have been ophitic, the plagioclase laths occurring as radial groups in a matrix of chlorite, calcite, and bleached amphibole. In most of the sections, however, no traces of diabasic structure can be detected. The rocks, though much decomposed, look as though they were composed of crystals, fragments, and small, almost dust-like grains of feldspar in a groundmass that yielded on decomposition new aWilliams, op. cit., p. 90. plagioclase, light-green amphibole, chlorite, and calcite. Most of the fragments are certainly portions of crystals bIbid., p.